The NOx Box

Hello again, Mateys!  Today we launched the second cast of the Niskins, and we are settling into a pattern for the nutrient experiments.  Andrew and I are ready at 4:00am for the cast, and while the Niskins are deployed, we ensure that everything is ready for the experiments – this is when we turn on our instruments, make the standards, and double check that we have every collection vial labeled properly.  Once the Niskins are recovered and our water samples collected, Andrew aliquots (distributes) the needed amount of water for each of the experiments and starts the ammonium analysis reaction (more in a future post!).  As he does this, I start creating the standard curve for the nitrate measurements using the standard nitrate concentrations we already made.

Remember, a standard solution is one in which concentration is known and is used to determine an unknown concentration.  This is why we first run the standards and make the curve – the regression (best-fit) line will tell us the nitrate concentrations of the unknown standards!

NOx Box standard curve for nitrate concentrations. We use the equation to calculate unknown nitrate concentrations!

NOx Box standard curve for nitrate concentrations. We use the equation to calculate unknown nitrate concentrations!

How do we use this curve to calculate the nitrate concentrations of the seawater samples?

So, you may be wondering, “What exactly is a NOx Box?”  Here’s a photo of the entire setup:

NOx Box setup. Everything needed to run the nitrate analysis!

NOx Box setup. Everything needed to run the nitrate analysis!

First of all, NOx is the sum of all gaseous nitrogen oxides (like NO and N2O).  The chemistry setup that you see on the right (the long glass tubes) produces the NO gas that the instrument detects through a reaction with ozone.  The green-colored liquid on the far right contains reduced vanadium (V3+) in an acidic solution that is heated to 90°C.  There is a small pink septum at the bottom of the vanadium tube – this is where the nitrate standards and samples are injected.  The nitrate reacts with the vanadium to make NO and oxidized vanadium according to the following reaction:                     [(aq) = aqueous, (g) = gas, (l) = liquid]:

3V3+ (aq) + NO3- (aq) + 4H+ (aq) –> 3V4+ (aq) + NO (g) + 2H2O (l)

The NO gas generated during the first reaction (and some HCl from the acidified solution) move to the second tube – this contains NaOH to scrub out the HCl:

NaOH (aq) + HCl (g) –> NaCl (aq) + H2O (l)

What type of reaction is this?  How do you know?

The NaOH reaction occurs in an ice bath to cool everything (it was heated to just below boiling!) before running through the instrument.  Now that HCl is removed, the NO gas can move into the NOx Box (the big tan machine in the middle) and react with ozone.  This reaction releases photons of light (energy) that the machine detects.

Once I’m done with the standards, I can inject the samples (the orange-capped tubes).  Whew! There’s a lot going on! Luckily, the machine does most of the work J.

So, back to my daily routine…  I’m finished running the nitrate samples by breakfast around 7:45am.  Between breakfast and lunch, Andrew and I work on the nitrite and ammonium experiments.  After lunch, I typically try to take a nap and relax so I’m refreshed for the end of the day, in which we clean and prep our collection bottles for the next day’s samples.

The days are very busy, but the science is a ton of fun!

Until next time,

Ms. B.


3 thoughts on “The NOx Box

  1. I wonder how the “Nox Box” reacts with the Ozone layer. From what you said, the solution reacts with the release of photons of light (energy) that the machine detects, so the “Nox Box” release energy when you put a solution inside the machine?

    • Excellent question, Michelle!

      The NOx Box itself doesn’t release the energy – that’s all part of the reaction between the gas and ozone. What the machine does is “see” how the amount of photons released – higher concentrations of the gas result in more light being released during the reaction. The NOx Box takes that amount of light and gives us a number to essentially say “that sample produced this amount of photons.”

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